Sessions lock - not using memcached?

Hello, I'm working with sessions in my PHP applications. My pages can take long time to be executed (~500ms) and I've simultaneous requests for the same session (Ajax requests). My problem is that I close sessions at the end of my PHP script, so my sessions files are locked for half a second. So, simultaneous requests (for the same session) can't be executed... simultaneously! My question is if I use memcached to store sessions, will my sessions be locked or can I access to them in the same time? If there aren't it should solve my problem. A second question is performances: is memcached sessions faster the PHP default sessions? I think yes because it doesn't make disk I/O..? Best regards, Samuel ROZE.